1. Field of the Invention
The invention relates to a method for passivating combustible, metallurgical dust which accumulates in the off-gas filter of crucible-drawing installations, in particular of crucible-drawing installations for drawing single silicon crystals. The invention also relates to an oxidation device for carrying out the method.
2. The Prior Art
The Czochralski crucible-drawing method is known for the production of high-purity single crystal rods. During the crucible-drawing of crystal rods, in particular of silicon rods using the Czochralski method, the monocrystalline or polycrystalline silicon fragments which are intended to produce the molten silicon are generally placed in a melting crucible, preferably in a quartz crucible. The silicon fragments are heated by means of a heat source, and then a monocrystalline seed is immersed in the molten liquid and drawn upward while being rotated, with the result that a crystal rod grows at the bottom end of the monocrystalline seed.
In order to establish the type of conductivity and the resistance of the crystal which is grown, dopants, such as boron, phosphorus, arsenic and antimony are added to the silicon fragments or the molten material. In addition to the silicon fragments and the dopants, the molten material also contains oxygen. The oxygen is introduced into the molten material from the surface of the quartz crucible by the contact between them. The oxygen is moved within the crucible by the forced convection of the molten material resulting from the rotation of the crucible and the heat convection of the molten material resulting from temperature differences therein.
During this movement, some of the oxygen is evaporated from the surface of the molten material in the form of the oxides of the constituents of the molten material. Depending on the particular dopant, these oxides, in addition to SiO, also include, owing to their high vapor pressure, the oxygen compounds of arsenic and antimony, in particular arsenic trioxide and antimony trioxide. Since an inert gas, preferably argon, generally flows continuously through the crucible-drawing installation, the oxides of the constituents of the molten material have different oxidation numbers in the gas phase and in the off-gas flow from the installation.
Finely divided metal oxide/semimetal oxide mixtures of medium oxidation numbers are highly reactive industrial dust materials with spontaneous combustion ignition temperatures of below 100.degree. C. These dust materials tend, in particular in the downstream off-gas filters, to burn the filters or to cause dust explosions.